The conversion of fossil fuels to power results in emission of a large amount of carbon dioxide into the atmosphere. Considering the vital role of fossil fuels to national economy, there is an urgent need to develop an effective methodology for carbon management. Carbon management involves steps including capturing, transporting, and securely storing carbon emitted from sources. The storing of carbon is a process known as carbon sequestration. Any viable system for sequestering carbon must be effective and cost-competitive, stable for long-term storage, and environmentally benign. Given the magnitude of carbon reductions needed to stabilize the atmosphere (nearly 1 gigaton of carbon/year by 2025 and 4 gigatons of carbon/year by 2050), CO2 capture and sequestration appears to be the most feasible method for reducing carbon emissions while maintaining the continued large-scale usage of fossil fuels.
There are various schemes such as geological sequestration, ocean disposal, mineral carbonation, and biological fixation that have been devised to reduce CO2 emissions to the atmosphere. Mineral sequestration is the reaction of CO2 with non-carbonate minerals to form geologically and thermodynamically stable mineral carbonates. Mineral carbonation is a new and less studied method of sequestration, which has a good potential to sequester a substantial amount of CO2. The reaction underlying mineral carbonation mimics natural chemical transformations of CO2, such as the weathering of rocks to form calcium or magnesium carbonates. This sequestration process provides a safe and permanent method of CO2 disposal. As CO2 is chemically incorporated into the mineral and is immobilized, it is unlikely that an accidental release of CO2 from the disposal site will occur. Furthermore, the reactions that bind CO2 to the mineral are exothermic and, if integrated efficiently, could result in CO2 disposal processes that are of net energy gains yielding a high economic viability.
The application of CO2 mineral sequestration has been visualized in two ways. The first process involves intermixing and reaction of minerals with CO2 in a process plant. The carbonation of minerals could take place either directly (gas-solid reaction) or in aqueous media (slurry carbonation). In the second process, CO2 could be injected into selected underground mineral deposits for carbonation (in-situ carbonation). The prior art suggests a number of possible approaches to achieve CO2 sequestration.
An early approach to mineral carbonation is taught by Pundsack, U.S. Pat. No. 3,338,667. Serpentine mineral (Mg6(OH)8Si4O10(Fe+2, Fe+3)) is reacted with ammonium bisulfate, a strong acid salt, to dissolve magnesium into solution. The pH of the solution is gradually raised to precipitate first iron oxide and then magnesium carbonate. However, the process is not designed to sequester carbon dioxide. Blencoe et al, U.S. Published Application 2004/0213705, teaches a process for CO2 sequestration which reacts a metal silicate with a caustic material to produce a metal hydroxide. The metal hydroxide is then contacted with a gas stream containing CO2 to produce a metal carbonate.
Other have attempted aqueous mineral carbonation processes. For example, one process has either heat treated serpentine or attrition ground olivine and serpentine to enhance the dissolution of those minerals for subsequent treatment with sodium bicarbonate. The energy requirements, however, for those pretreatment options (heating and/or external grinding) are extremely high. Other have used strong acids to dissolve the minerals. However, the costs of acid recovery and post-treatment of recovered solids will be expensive. Accordingly, there remains a need in this art for a CO2 sequestration process that is effective and cost-competitive, stable for long-term storage, and environmentally benign.